Human immunodeficiency virus type 1 (HIV-1) requires CD4 and a coreceptor for entry, into susceptible host cells. The chemokine receptors CXCR4 and CCR5 are the major coreceptors used by T cell line-tropic (X4) and macrophage-tropic (R5) HIV-1 isolates. A naturally occurring frame-shift mutation caused by a 32 base pair deletion (delta32) in the human CCR5 gene resulted in a truncated protein that does not function as a coreceptor. Relative to the general population, CCR5delta32 homozygotes (-/-) are rarely found among HIV-1 infected individuals. In addition, HIV-1 infected CCR5delta32 heterozygotes (+/-) progress more slowly to AIDS than individuals lacking this allele indicating that even one delta32 allele confers some resistance. We have found that expression of full-length delta32 protein in human cell lines or normal human peripheral blood mononuclear cells (PBMCs) can specifically downmodulate endogenous CXCR4 and inhibit X4 infection. We have demonstrated efficient co- localization of delta32 and either CXCR4 or CCR5. Truncation of the delta32 C-terminus resulted in the loss of CXCR4 downmodulation activity. Furthermore, we showed that PBMCs from several (-/-) individuals express lower CXCR4 levels in comparison to wild-type CCR5 (+/+) PBMCs. We hypothesize that one mechanism of resistance to HIV-1 is caused by the unique activity of the delta32 protein. Using delta32-specific antibodies, we have shown that native delta32 protein was expressed in (-/-) PBMCs isolated from exposed/uninfected [1]but was absent in an infected (-/-) individuals [2], indicating a critical role for the delta32 protein in resistance to HIV-1 infection. The overall purpose of this proposal is to determine the mechanism(s) that underlies the observed more broad protective effect by the naturally occurring delta32 protein. The following specific aims are based on preliminary data generated in my laboratory: 1. Analyze how delta32 impairs functional expression of CCR5 and CXCR4. 2. Analyze the mechanism of failure of delta32 protective effect in an infected delta32 homozygous individual. 3. Determine the structural determinants involved in delta32 activity.